In propeller assemblies for aircraft power plants, undesirable vibrations can be set up by various external forces and transmitted to the body or fuselage of the aircraft. In helicopters, for example, external aerodynamic forces can and do produce various motions of the rotor blades relative to the drive shaft, including lead-lag movements of the blades (i.e. oscillations of the blades in their plane of rotation relative to the rotor hub or drive shaft). Such movements represent a primary source of undesirable vibrations.
Proposals to reduce the generation and/or transmission of vibrations from aircraft propellers to aircraft fuselages have included: (a) specially designed blades; (b) articulating, rather than rigid, connections between propeller blades and their propeller hub; and (c) elastic connections between propeller hubs and their drive shafts. The use of articulating connections for individual blades has gained considerable favor, particularly in the helicopter industry. Early designs for individually articulated blades utilized pin-type hinge connectors to permit various motions of the blades, including in-plane or lead-lag movements, and also incorporated damping mechanisms to control the frequency and amplitude of such movements. One design, utilizing hinge-type connectors and friction-type damping mechanisms, is described and illustrated in Campbell U.S. Pat. No. 2,494,985. More recent designs utilize high capacity, laminated elastomeric bearings to provide the necessary articulation, as exemplified by the rotor system described and illustrated in Rybicki U.S. Pat. No. 3,782,854. The damping mechanisms may incorporate viscoelastic material, such as rubber, as exemplified by the mechanisms of Potter U.S. Pat. No. 3,758,230 and Potter U.S. Pat. No. 3,842,945.
In proposals for resiliently connecting a propeller hub to a drive shaft, the propeller blades are all connected to a hub and elastomeric material is interposed between the drive shaft and the hub. Such a propeller assembly is described and illustrated in Julien et al U.S. Pat. No. 2,312,822. In the propeller assembly illustrated in FIG. 12 of the Julien et al patent, for example, a metal plate is secured to an end of the drive shaft in a plane that is generally perpendicular to the shaft. A second metal plate is secured to the propeller hub parallel to and spaced axially from the first plate. A body of elastomer is interposed between the plates and bonded to both plates. Rotational movement of the drive shaft is transmitted to the blades through the elastomer. While the opposed blades of the Julien propeller are essentially rigidly interconnected, a pair of opposed propeller blades may be flexibly interconnected using an intermediate flexible strap, as suggested for helicopter rotors in Mautz U.S. Pat. No. 3,578,877 and Baskin U.S. Pat. No. 3,874,815.
Inherent with the lead-lag movement of rotor blades are scissor-like or "scissors mode" movements between adjacent blades. To a large extent, "scissors mode" motions in rotors having individually articulated blades can be controlled by lead-lag dampers for the individual blades. In rotors utilizing pairs of interconnected blades, where "scissors mode" movements occur between the blades of different blade pairs, this inventor is not aware of any specific proposals to control such movements. Both lead-lag and "scissors mode" movements are more pronounced and more likely to occur in helicopter rotors, where the air flow to the blades is generally radial, than in propellers for fixed wing aircraft, for example.